Re: [PATCH v3] mm/page_alloc: bail out on fatal signal during reclaim/compaction retry attempt

[Matthew Wilcox]

On Thu, May 20, 2021 at 12:42:57PM +0100, Aaron Tomlin wrote:
> On Thu 2021-05-20 12:20 +0200, Vlastimil Babka wrote:
> > On 5/20/21 6:34 AM, Andrew Morton wrote:
> > > 
> > > What observed problems motivated this change?
> > > 
> > > What were the observed runtime effects of this change?
> > 
> > Yep those details from the previous thread should be included here.
> 
> Fair enough.
> 
> During kernel crash dump/or vmcore analysis: I discovered in the context of
> __alloc_pages_slowpath() the value stored in the no_progress_loops variable
> was found to be 31,611,688 i.e. well above MAX_RECLAIM_RETRIES; and a fatal
> signal was pending against current.

While this is true, it's not really answering Andrew's question.
What we want as part of the commit message is something like:

"A customer experienced a low memory situation and sent their task a
fatal signal.  Instead of dying promptly, it looped in the page
allocator failing to make progress because ..."

> 
>      #6 [ffff00002e78f7c0] do_try_to_free_pages+0xe4 at ffff00001028bd24
>      #7 [ffff00002e78f840] try_to_free_pages+0xe4 at ffff00001028c0f4
>      #8 [ffff00002e78f900] __alloc_pages_nodemask+0x500 at ffff0000102cd130
> 
>                                                              //      w28 = *(sp + 148)      /* no_progress_loops */
>      0xffff0000102cd1e0 <__alloc_pages_nodemask+0x5b0>:      ldr     w0, [sp,#148]
>                                                              //      w0 = w0 + 0x1
>      0xffff0000102cd1e4 <__alloc_pages_nodemask+0x5b4>:      add     w0, w0, #0x1
>                                                              //      *(sp + 148) = w0
>      0xffff0000102cd1e8 <__alloc_pages_nodemask+0x5b8>:      str     w0, [sp,#148]
>                                                              //      if (w0 >= 0x10)
>                                                              //          goto __alloc_pages_nodemask+0x904
>      0xffff0000102cd1ec <__alloc_pages_nodemask+0x5bc>:      cmp     w0, #0x10
>      0xffff0000102cd1f0 <__alloc_pages_nodemask+0x5c0>:      b.gt    0xffff0000102cd534
> 
>  - The stack pointer was 0xffff00002e78f900
> 
>      crash> p *(int *)(0xffff00002e78f900+148)
>      $1 = 31611688
> 
>      crash> ps 521171
>         PID    PPID  CPU       TASK        ST  %MEM     VSZ    RSS  COMM
>      > 521171      1  36  ffff8080e2128800  RU   0.0 34789440  18624  special
> 
>      crash> p &((struct task_struct *)0xffff8080e2128800)->signal.shared_pending
>      $2 = (struct sigpending *) 0xffff80809a416e40
> 
>      crash> p ((struct sigpending *)0xffff80809a416e40)->signal.sig[0]
>      $3 = 0x804100
> 
>      crash> sig -s 0x804100
>      SIGKILL SIGTERM SIGXCPU
> 
>      crash> p ((struct sigpending *)0xffff80809a416e40)->signal.sig[0] & 1U << (9 - 1)
>      $4 = 0x100
> 
> 
> Unfortunately, this incident was not reproduced, to date.
> 
> 
> 
> 
> 
> Kind regards,
> 
> -- 
> Aaron Tomlin